The objectives of the proposed research are to characterize kinetically and pharmacologically some of the active transport systems in the choroid plexus of the lateral ventricles that are responsible for the formation of the CSF and for the maintenance of the constancy of the composition of this fluid. The present studies include the role of the choroid plexus in regulating CSF lithium concentration, the interaction of Li, Na, K, at the level of the inorganic cation transport system of the choroid plexus, the nature of the Na transport system(s), and the effects of acetazolamide and furosemide of CSF secretion by the choroid plexus.